A meteor and asteroid: 1 in 100 million odds

By Meg Urry, Special to CNN

Updated 8:16 PM ET, Mon February 18, 2013

All about asteroids19 photos

All about asteroids – Asteroid Day, a day organized by a group of notable scientists, astronauts and citizens, aims to educate the global community on the potential threat of asteroids on June 30. Click through the gallery to learn more about these Near-Earth Objects.

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All about asteroids – This graphic shows the track for asteroid 2004 BL86, which flew about 745,000 miles from Earth on Monday, January 26. That's about three times as far away as the moon.

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This graphic shows the path Asteroid 2014 RC took as it passed Earth on September 7. The space rock came within one-tenth the distance from Earth to the moon.

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NASA scientists used Earth-based radar to produce these sharp views of the asteroid designated "2014 HQ124" on June 8. NASA called the images "most detailed radar images of a near-Earth asteroid ever obtained."

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The Hubble Space Telescope snapped a series of images on September 10, 2013, revealing a never-before-seen sight: An asteroid that appeared to have six comet-like tails.

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A diagram shows the orbit of an asteroid named 2013 TV135 (in blue), which made headlines in September 2013 when it passed close by Earth. The probability of it striking Earth one day stands at 1 in 63,000, and even those odds are fading fast as scientists find out more about the asteroid. It will most likely swing past our planet again in 2032, according to NASA.

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Asteroid 2012 DA14 made a record-close pass -- 17,100 miles -- by Earth on February 15, 2013. Most asteroids are made of rocks, but some are metal. They orbit mostly between Jupiter and Mars in the main asteroid belt. Scientists estimate there are tens of thousands of asteroids and when they get close to our planet, they are called near-Earth objects.

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Another asteroid, Apophis, got a lot of attention from space scientists and the media when initial calculations indicated a small chance it could hit Earth in 2029 or 2036. NASA scientists have since ruled out an impact, but on April 13, 2029, Apophis, which is about the size of 3½ football fields, will make a close visit -- flying about 19,400 miles (31,300 kilometers) above Earth's surface. The images above were taken by the European Space Agency's Herschel Space Observatory in January 2013.

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If you really want to know about asteroids, you need to see one up close. NASA did just that. A spacecraft called NEAR-Shoemaker, named in honor of planetary scientist Gene Shoemaker, was the first probe to touch down on an asteroid, landing on the asteroid Eros on February 12, 2001. This image was taken on February 14, 2000, just after the probe began orbiting Eros.

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The first asteroid to be identified, 1 Ceres, was discovered January 1, 1801, by Giuseppe Piazzi in Palermo, Sicily. But is Ceres just another asteroid? Observations by NASA's Hubble Space Telescope show that Ceres has a lot in common with planets like Earth. It's almost round and it may have a lot of pure water ice beneath its surface. Ceres is about 606 by 565 miles (975 by 909 kilometers) in size and scientists say it may be more accurate to call it a mini-planet. NASA's Dawn spacecraft is on its way to Ceres to investigate. The spacecraft is 35 million miles (57 million kilometers) from Ceres and 179 million miles (288 million kilometers) from Earth. The photo on the left was taken by Keck Observatory, Mauna Kea, Hawaii. The image on the right was taken by the Hubble Space Telescope.

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One big space rock got upgraded recently. This image of Vesta was taken by the Dawn spacecraft, which is on its way to Ceres. In 2012, scientists said data from the spacecraft show Vesta is more like a planet than an asteroid and so Vesta is now considered a protoplanet.

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The three-mile long (4.8-kilometer) asteroid Toutatis flew about 4.3 million miles (6.9 million kilometers) from Earth on December 12, 2012. NASA scientists used radar images to make a short movie.

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Asteroids have hit Earth many times. It's hard to get an exact count because erosion has wiped away much of the evidence. The mile-wide Meteor Crater in Arizona, seen above, was created by a small asteroid that hit about 50,000 years ago, NASA says. Other famous impact craters on Earth include Manicouagan in Quebec, Canada; Sudbury in Ontario, Canada; Ries Crater in Germany, and Chicxulub on the Yucatan coast in Mexico.

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NASA scientists say the impact of an asteroid or comet several hundred million years ago created the Aorounga crater in the Sahara Desert of northern Chad. The crater has a diameter of about 10.5 miles (17 kilometers). This image was taken by the Space Shuttle Endeavour in 1994.

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In 1908 in Tunguska, Siberia, scientists theorize an asteroid flattened about 750 square miles (1,200 square kilometers) of forest in and around the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia.

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What else is up there? Is anyone watching? NASA's Near-Earth Object Program is trying to track down all asteroids and comets that could threaten Earth. NASA says 9,672 near-Earth objects have been discovered as of February 5, 2013. Of these, 1,374 have been classified as Potentially Hazardous Asteroids, or objects that could one day threaten Earth.

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One of the top asteroid-tracking scientists is Don Yeomans at the Jet Propulsion Laboratory, which is managed by the California Institute of Technology. Yeomans says every day, "Earth is pummeled by more than 100 tons of material that spewed off asteroids and comets." Fortunately, most of the asteroid trash is tiny and it burns up when it hits the atmosphere, creating meteors, or shooting stars. Yeomans says it's very rare for big chunks of space litter to hit Earth's surface. Those chunks are called meteorites.

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Asteroids and comets are popular fodder for Earth-ending science fiction movies. Two of the biggest blockbusters came out in 1998: "Deep Impact" and "Armageddon." (Walt Disney Studios) Others include "Meteorites!" (1998), "Doomsday Rock" (1997), "Asteroid" (1997), "Meteor" (1979), and "A Fire in the Sky" (1978). Can you name others?

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Asteroid 1998 QE2 is about 3.75 million miles from Earth. The white dot is the moon, or satellite, orbiting the asteroid.

The asteroid, named DA14, came within 17,000 miles or so, as close as a telecommunication satellite in geosynchronous orbit. DA14 is quite a bit smaller than YU55, the asteroid that passed Earth in November 2011, but DA14 came more than 10 times closer.

These two rare events occurred the same day. Your inner mathematician and your inner prophet of the end times think they should be connected. But scientists say they are not. What gives?

First, some facts. Meteors are rocky bodies that enter the Earth's atmosphere. Some are leftover debris out of which planets like Earth are formed, while others are the remnants of shattered comets and asteroids. As long as their orbit intersects the Earth's orbit, these rocks can in principle impact the Earth.

Meg Urry

Actually, this happens all the time, although usually the impacts occur in unpopulated regions since most of Earth is uninhabited. In fact, most meteors fall into the ocean simply because water covers two-thirds of the planet.

So we don't witness most meteor impacts. If one landed in New York City or Moscow, people would definitely notice. Fortunately, the odds are very much against hitting a densely populated region.

The meteor that fell Friday near Chelyabinsk, Russia, was pretty big, maybe 50 feet across. In 1908, a slightly larger meteor -- perhaps three times larger in diameter, or 27 times larger in mass -- flattened a thousand square miles of forest near Tunguska, Russia, downing some 80 million trees.

NASA scientists estimate that meteors as large as Friday's might hit the Earth every decade or two, while Tunguska-like events are estimated to occur once every 1,000 years.

Photos: Meteor explodes over Russia 8 photos

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Meteor explodes over Russia – A large chunk of a meteor that exploded over Russia is found in a lake on Friday, February 15.

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Meteor explodes over Russia – A meteor streaks through the sky before exploding with a flash and boom that shattered glass in buildings and left hundreds of people hurt.

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Meteor explodes over Russia – Workers repair a damaged power line near the wall of a local zinc plant. About 270 buildings were damaged -- mostly broken glass -- by shock waves caused by the blast, said Vladimir Stepanov of the National Center for Emergency Situations at the Russian Interior Ministry.

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Meteor explodes over Russia – The meteor leaves a white streak through the sky. The national space agency, Roscosmos, said scientists believed one meteoroid had entered the atmosphere, where it burned and disintegrated into fragments, according to RIA Novosti.

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Meteor explodes over Russia – The meteor's vapor trail passes over the city.

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Meteor explodes over Russia – A man removes shards of glass from the frame of a broken window.

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Meteor explodes over Russia – The meteor damaged windows at a sports hall.

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Meteor explodes over Russia – By noon Russia time more than 725 people had sought medical help.

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The close fly-by of an asteroid like DA14, like the Tunguska meteor, is a once-in-1,000-years event. Asteroids are large, irregular, rocky bodies orbiting the Sun roughly between Mars and Jupiter. Many have impacted the Earth over its 4.5 billion-year history --as they have hit the moon, Mars and other planets -- leaving craters behind.

A particularly large asteroid -- roughly 300 times larger across than DA14 (and 30 million times its volume, and far more rare) -- created a planetary extinction event that did in dinosaurs 65 million years ago, allowing mammals to rise to their present-day prominence.

Using NASA's WISE infrared satellite, astronomers estimate there are about 5,000 known meteors that can impact the Earth with sizes of about 100 feet or larger -- that is, larger than the Chelyabinsk meteor. Smaller ones are fainter and thus harder to find.

It makes sense that smaller asteroids pass Earth more frequently and, on average, closer. That's because in nature, small things are more common than big things. So asteroids like YU55 are more rare than DA14, which in turn is more rare than the Chelyabinsk meteor. Because there are more DA14s filling interplanetary space than YU55s, a 50-foot asteroid can be found in a smaller volume of space, on average, and thus closer to Earth, than a 150-foot one.Now let's talk about coincidence. Mathematicians frame this issue in terms of probability -- that is, the likelihood that something will happen. A rare thing is unlikely, so we say it has a low probability of occurring.

Two rare events happening at approximately the same time is much more unlikely. Here is how to think of it mathematically: If the events are not associated, the probability of this coincidence comes from multiplying the individual probabilities.

For example, the probability that your birthday is on a given date -- say, January 1 -- is 1/365. That is, of every 365 readers of this article, roughly one will have a birthday on January 1.

Now, the probability that the next reader's birthday is also on January 1 is 1/365 times 1/365, or about 1 in 130,000. If that many people read the article, such a coincidence could happen. Of course, it's much more likely that two non-consecutive readers will have a birthday on January 1. And it's very likely that lots of readers have the same birthday as other readers. (In fact, in any group of 23 or more people, it is more than 50% likely that two will share a birthday, but calculating that probability is a bit more complicated.)

Back to the meteor and the asteroid. Both events happening within one day makes us think they could be connected. That instinct comes from doing the math -- if it is improbable, then we think it cannot be a coincidence.

But the facts don't support this conclusion. First of all, in the time between the two events, the Earth moved roughly 300,000 miles, meaning the asteroid and the meteor were in completely different places. Moreover, they traveled in completely different directions, so they couldn't have been associated.

So there is no way the meteor and the asteroid are connected. It has to be a coincidence that the two events happened on the same day. Yet this would seem to be at odds with our instinct that two very rare things would not happen at the same time.

How can we reconcile these two opposite thoughts: the impossibility of an association based on the physics of trajectories, and the improbability of coincidence (lack of association) that the math suggests?

The answer is that we need to rethink the probability calculation. If asteroids as big as DA14 pass close to Earth once every decade or two, and meteors as large as the Chelyabinsk one impact once every 100 years (a similar meteor having caused the Tunguska event in 1908), the chance of both events happening on any one day are indeed very small: 1 in 3,650 days times 1 in 36,500 days, or about 1 in 100 million -- not odds you would bet against.

But think again: The Earth has been around for 4.5 billion years -- which is 1.6 trillion days. So the chance that these two events would happen on a day sometime in the earth's history is actually larger than we first thought -- it ought to have happened about 12,000 times already.

Of course, during most of that 4.5 billion year history, the earth was not populated by intelligent life -- human beings who might have noticed the two events happening on the same day.

So what is the probability that the meteor hits and the asteroid passes Earth on the same day when someone could record it on video? That's probably been possible for about 50 years, or only about five years if we have to do it on a smartphone or dashboard camera. That's 1,825 days, which means the chance of someone filming the event is only about one in 70,000 -- and that's if people blanketed the Earth. Given how sparsely the Earth is populated, we should correct this number downward by a (large!) geographical factor. It's also unlikely that this event would happen within 3,000 miles of the Tunguska impact.

What to think? Our rough calculation says a large meteor impact on the same day as closest passage of the DA14 asteroid is really improbable. But it did happen. Something in our assumptions could be wrong. For example, the frequency of meteor impacts could be much larger and our estimates too low because we don't notice most of them.